The precise measurement of cholesterol absorption involves a number of uncertainties which must be answered by the student of cholesterol metabolism. Recently, a new and simple procedure, the isotope-ratio method (IRM), has been proposed for the measurement of cholesterol absorption involving simultaneous administration of oral and intravenous labeled cholesterol; results can be calculated from the analysis of plasma rather than of fecal radioactivity. We have compared the results of this method in 14 hospitalized patients to a method involving fecal analysis (Method IV), and found good agreement. In the present project we propose to further validate the IRM by comparing it to Method IV in the following experimental series and to address the uncertainties which may plague the precision of these measurements: 1) We will measure the effect of intraluminal isotope exchange on the results. We will attempt to differentiate between the effects of isotope-exchange and measure and compare the absorption of exogenous (dietary) and endogenous (biliary) cholesterol. 3) We will compare the IRM to Method IV in the following circumstances: a) we will vary the time of the oral tracer administration; b) we will vary the time blood samples are taken for radioactivity analysis; c) we will administer the oral tracer in multiple doses. 4) We will work out and perfect a questionnaire method to estimate daily cholesterol intake of out-patients, so that results of cholesterol absorption can be expressed in terms of absorbed mass rather than percent absorption.